This invention is directed to hand-held guides used with hand-held power saws. Such saws typically have motor driven circular blades mounted on an adjustable plate. The plate can be moved up and down to adjust the depth of the saw cut and can be rotated up to 45.degree. in a direction perpendicular to the plane of the blade and creating either square or angular saw cuts. The blade protrudes through a slot in the plate which on most such saws is located approximately 3 to 4 inches from one side and 11/2 to 2 inches from the other side of the plate. Most saws are designed for righthand use, with the motor mounted to the left side of the plate and the 4-inch distance between slot and side also located to the left.
In normal use, a line is drawn with a straightedge. The user must then visually align the saw blade with the line from the start of cut throughout the cutting process. Such a procedure is dangerous, and the accuracy of the cuts is limited to the ability of the user.
Persons who desire to make consistently straight cuts use various types of straightedge devices as a guide against which the edge of the saw plate can be moved. A number of such guides are available, some of which have fixed angles and others of which can be varied to produce different angles of cut. However, since the saw blade is offset from the sides of the plate, the straightedges have to be positioned varying distances away from the desired line of cut, with the distances varying as the angle of cut is changed, as the blade is rotated with respect to the plate, from saw-to-saw due to varying locations of the slot, with different saw blades with varying cut widths, etc.
One example of a power saw guide is disclosed in U.S. Pat. No. 4,054,077 to Gram. The Gram device includes a base plate 12 in the form of an isosceles triangle having two sides of equal and opposite angle for cutting identical "negative" and "positive" acute anges. Transverse to the angled sides is an index member 14 which can be held against the near edge of the member to be cut. The power saw base plate can be run along either of the angled sides of the saw guide base plate 12, and in doing so the index 14 will be cut off parallel to the edge of the saw guide base plate 12 and at a distance indicating the location of the cut. In all future cutting operations with the same saw, the edge of the index 14 will serve as a cut location indicator. There are a number of problems characteristic of the Gram device. For example, the lower surface of the saw guide base plate 12 will be flush with the upper surface of the board to be cut, but the index member 14 is embedded in a groove in the lower surface of the base plate 12, with the result that the upper surface of the index member 14 will be at a higher level than the upper surface of the material being cut. Due to this raised index, the saw guide must always be placed on the near side of the board, since the base plate of the power saw itself will collide with the index member 14 if the member 14 is placed on the far side of the board. Accordingly, the guide member of Gram must be rotated through 180.degree. in order to cut both acute angles, and must be flipped upsidedown to cut the square angle along the edge which forms the base of the isosceles triangle.
Further, the index 14 is at an angle with respect to the guide 18. The direction of rotation of the saw blade, by design, pulls the bottom of the saw's platform down against the surface of the material being cut. As the saw is passed over the edge 20 and along line 48, the front right corner of the power saw base plate will be pulled downward toward the material being cut, while the raised level of the index 14 in the Gram saw guide device will keep the left-front corner of the power saw base plate at a higher level, thereby torquing and binding the saw blade.
Further, with regard to the grooved design illustrated in FIGS. 3-7 of the Gram patent, the groove will keep the left side of the power saw platform at the level of the index, and once again the natural forces of the saw will pull down the right front corner of the power saw platform, thereby changing the plane of the cut. This is even true of the square cut made with index 132 in FIG. 7.
As described in the Gram patent, that device is designed for use on a flat workbench in which there are dowels to hold the device in a fixed position. Its large size, unique shape, etc. render it unsuitable for a number of common power saw applications, e.g., the cutting of rafters in the field using saw horses for support.
Other saw guide devices are disclosed in: U.S. Pat. No. 1,568,115 to Weaver; U.S. Pat. No. 2,581,766 to McConnell; U.S. Pat. No. 2,625,184 to Harbert; U.S. Pat. No. 2,668,347 to Schmidt; U.S. Pat. No. 2,719,548 to Mitchell; U.S. Pat. No. 2,735,445 to Forsberg; U.S. Pat. No. 2,773,523 to Hopla; U.S. Pat. No. 4,179,965 to Johnson; and U.S. Pat. No. 4,281,572 to Stovall. These references generally teach a number of variable angle saw guides, but all have been unsatisfactory for a number of reasons. One feature common to all of these conventional variable-angle saw guides is that the pivot point of the saw guide is a single, non-variable and coincident point of rotation, i.e., a hole drilled in both the fixed member and the saw guide through which a single bolt or rivet is passed. The pivot point cannot be placed along the cut line, since a pivot pin would then be in the path of the saw blade. Accordingly, the pivot point is offset from the line of the cut. If the fixed member of such a saw guide is clamped to a saw board and a number of saw cuts are made while varying the saw guide through its entire range of angles, not only the angle of the cut but also the position thereof will be varied. Thus, upon varying the cut angle, the operator cannot determine where the cut will end up.
Since conventional variable-angle guide devices do not provide for a common point of intersection for all cuts regardless of angle, each has found it necessary to attach its saw cut location index to the movable portion of the guide, rather than to the fixed member. For example, U.S. Pat. No. 2,719,548 to Mitchell discloses a cumbersome cut location indicator device which is as long as the saw guide itself. As illustrated in FIG. 1 thereof, two hinges 11 are necessary to pivot the indicator out of the way prior to making a cut.
U.S. Pat. No. 2,772,523, to Hopla discloses a simple wire-type indicator which is also attached to the guide. The design of the Hopla device is such that it cannot be sufficiently repetitive and must also be rotated out of the way.
U.S. Pat. No. 4,179,965 to Johnson discloses a saw guide having an adjustable cutting angle which can also accommodate power saws having different blade offsets. In the Johnson device, the width of the guide can be adjusted to be equal to the blade offset of the saw. The guide is first placed with its outside edge located on the desired line of cut, a pencil mark is made along the inside edge of the guide, and the guide is then re-positioned so that its outside edge is along the new mark. This is a cumbersome operation.
The use of conventional saw guide devices is often aggravating, and few power saw operators utilize straightedges despite the loss of accuracy and the significant increase in danger, e.g., if the saw is not passed across the material in a straight line, the blade tends to bind and run backwards into the operator. Hand-held power saws are perhaps the most dangerous tool utilized in normal carpentry, and there is a great need for a simple, easy-to-use and effective power saw guide which will be more readily used by power saw operators.